1. Field of the Invention
The present invention relates to a current detection apparatus to detect driving battery charging current and discharging current in a car power source apparatus.
2. Description of the Related Art
In a car power source apparatus, it is important to accurately compute remaining capacity of the driving battery. If error develops in detecting remaining battery capacity, that error accumulates as driving time increases. Cumulative error, which is the difference in remaining capacity between actual remaining battery capacity and computed remaining battery capacity, can become a cause of battery over-charging and battery over-discharging. Battery electrical characteristics degrade markedly with over-charging and over-discharging leading to shorter battery lifetime. Since a car driving battery is extremely expensive, it is important to enable its use for as long a period as possible. Further, in limiting the range of remaining battery capacity to avoid over-charging and over-discharging, the usable range narrows as the cumulative error increases resulting in vehicle driving limitations.
Remaining battery capacity is computed by integrating current flowing through the battery. Remaining capacity is computed by subtracting the integrated value of discharging current from the integrated value of charging current while considering charging efficiency and discharging efficiency. To accurately compute remaining capacity, it is necessary to accurately detect battery current. Here, an analog signal detected by a battery current sensor is converted to a digital signal by an analog-to-digital (A/D) converter, and that digital signal is operated on by a current correction circuit to compute remaining capacity. The current detection apparatus detects battery discharging current and charging current with a current sensor. The analog signal output from the current sensor is converted to a digital signal by an A/D converter and input to a computing circuit to compute remaining capacity. FIG. 1 shows the output characteristics of a current sensor. In a current sensor with these characteristics, output voltage increases as discharging current increases, and output voltage decreases as charging current increases. The current sensor of FIG. 1 detects 200 A of charging current and discharging current and outputs a voltage proportional to the current. Specifically, this current sensor detects change in current over 400 A and outputs a voltage signal from 0V to 10V.
An output signal from the current sensor is input to the A/D converter and the digital signal output from the A/D converter is operated on by the computing circuit to detect battery current. With a detected current of 0 A as its center-point, the current sensor detects charging currents and discharging currents having the same maximum current value. However, under actual battery operating conditions, maximum charging current and maximum discharging current are not equal. Battery charging current and discharging current are controlled by the battery ECU, and maximum charging current is controlled to a smaller value than maximum discharging current. This is to optimize driving conditions while protecting the battery. By allowing discharging current that is as large as possible, desired performance such as accelerating power can be offered to the driver. However, if a large amount of charging current is supplied to the battery, there is concern that a rapid rise in battery charging voltage could result. If the battery is over-charged to the point where the specified voltage of the system is exceeded, not only is battery lifetime reduced, but in the worst-case, safety concerns are introduced such as battery electrolyte leakage. Consequently, a current detection circuit that detects the same range for charging current and discharging current cannot detect current with high precision. This is because the precision of 1-bit of digital signal converted by the A/D converter is coarse in such a system. For example, an apparatus that converts current in a 400 A range to a 10-bit digital signal has a 1-bit precision of 400/1024 A.
As an apparatus to resolve these drawbacks, the present inventor previously developed a detection apparatus with charging current and discharging current switching to detect current with high precision. Details of that apparatus are cited in Japanese Laid-Open Patent Publication No. 2002-62341.